The present invention relates to an exposure method for transferring an image of a pattern formed in a mask onto a photosensitive substrate through a projection optical system.
As an exposure apparatus for transferring an image of a pattern formed in a mask onto a photosensitive substrate, various types of apparatus have been developed. For example, as a projection exposure apparatus for production of a liquid crystal display (LCD), many step-and-repeat type apparatus (hereinafter, referred to as "stepper") are being used. In such a stepper, after a predetermined area of a substrate is exposed to a pattern of an LCD formed in a mask, a stage on which the substrate is placed is shifted a predetermined distance in a stepping manner to perform exposure of another area of the substrate. Such an operation is repeatedly performed on the entire area on the substrate to complete exposure of the entire substrate. Normally in this stepper, an LCD pattern is formed by a picture composition method. That is, four exposed patterns 100a, 100b, 100c, 100d on a substrate are combined, as indicated in FIG. 13, thereby forming a single LCD pattern on the substrate. Each of the patterns 100a, 100b, 100c, 100d corresponds to a different mask. That is, the patterns 100a, 100b, 100c, 100d are sequentially formed by exposure while one of the four masks is sequentially replaced by another mask. The four patterns are then combined into an LCD.
In exposure using such a picture composition method, a positional deviation of a transferred pattern 112 may sometimes occur as indicated in FIGS. 14(a)-(c). In the drawings, dotted lines indicate a desired transfer position, and solid lines indicate transferred positions that have errors. As positional deviations at the time of pattern transfer, there are: (a) a positional deviation wherein a transferred pattern 112 has a shift error; (b) a positional deviation wherein a transferred pattern 112 has a rotational error; and (c) a positional deviation wherein a transferred pattern 112 has a magnification error. Since an LCD is formed normally by superimposing about seven to eight layers on a glass substrate, an error as mentioned above can be a cause for an overlay deviation between layers. In a TFT (thin film transistor) LCD, a positional deviation at a stitching 110 (see FIG. 13) or an overlay deviation between layers degrades characteristics of transistors and causes defects such as color irregularity or the like. As factors that cause errors as indicated in FIGS. 14(a)-(c), optical aberrations, such as distortion, in the projection optical system, mask alignment errors, mask patterning errors, and the like can be considered.
Several methods for solving these problems have now been proposed. For example, as shown in FIG. 15, a plurality of coordinate measurement marks (126a, 126b, 126c, 126d, 126e, 126f, 126g, 126h) are arranged around an LCD pattern 122 within an effective exposure area 124 on a mask 120, and coordinates of the images transferred from the coordinate measurement marks (126a-126h) onto a substrate through a projection optical system are measured using a predetermined position detection device provided in the apparatus. From the amount of deviation between measured values and desired values of coordinates where these marks (126a-126h) are transferred, a correction value (a mask shift, a mask rotation, a magnification) that minimizes the amount of deviation is calculated by means of, for example, least square approximation or the like. After that, using the thus-calculated correction value, the mask 120 is precisely aligned, and then exposure is performed. Such a method can optimize a positional deviation of transfer caused by a patterning error of the mask 120 and an optical aberration of the projection optical system, and can reduce the positional deviation at stitchings and the overlay deviation between layers.
In the conventional methods as described above, however, there is a possibility that when coordinates, on a substrate, of the images of the coordinate measurement marks (126a-126h) in the mask 120 are detected by a position detection device, a deviation may occur between a detected coordinate and the actual transferred position on the substrate. This problem becomes serious especially in a case where images of reference marks provided on a stage, at substantially the same height as a substrate surface, are reversely projected onto a mask through a projection optical system, and coordinates at which the reference mark images superimpose on the coordinate measurement marks (126a-126h) are measured by a predetermined sensor, which coordinates are determined as positions at which the coordinate measurement marks (126a-126h) are transferred onto the substrate. More specifically, variations in optical aberrations, such as curvature of image field and the like, due to variations in the projecting direction in the projection optical system, cause deviations between imaginary transfer positions determined by the above-described method and the actual transferred positions on the substrate. Therefore, an error becomes included in a correction value that is calculated from a difference between a measured value of mask position provided by the position detection device and a desired value of position where an image is transferred, thereby making it impossible to properly correct the mask position, the magnification of the projection optical system, or the like, at the time of exposure. As a result, the positional deviation at stitchings of the exposed LCD screens or the overlay deviation between layers is not improved, and may be degraded.